For removal of nitrogen oxides in an oxygen-containing exhaust gas, such as that of an internal combustion engine operated with excess air, especially from a diesel engine, catalysts based on titanium dioxide are known, over which the nitrogen oxides are reduced in the presence of oxygen by means of an added reducing agent, such as ammonia in particular, to give molecular nitrogen and water. In this regard, reference is made by way of example to DE 24 58 888 A1. In this process for selective catalytic reduction, SCR process for short, the reducing agent or a precursor substance which is converted to the reducing agent in the exhaust gas is added to the exhaust gas prior to entry into the catalytic converter. For example, a known precursor substance for the reducing agent ammonia is urea, which is supplied to the exhaust gas especially in the form of an aqueous urea solution. Also known as alternative reducing agents are hydrocarbons which, especially in the case of incomplete combustion in the internal combustion engine, may already be present as combustion products in the exhaust gas.
The SCR-active catalysts known from DE 24 58 888 A1 comprise a ceramic catalyst composition comprising titanium dioxide as the main constituent, with additions of oxides of tungsten and/or vanadium. The catalyst bodies used here may be coated catalysts or unsupported catalysts. In the coated catalysts, the catalyst composition has been applied to a support material, such as more particularly to a cordierite (a magnesium aluminosilicate of the composition Mg2Al4Si5O18 with rhombic dipyramidal structure) which is itself catalytically inactive. An unsupported catalyst, in contrast, is manufactured entirely from the catalytically active catalyst composition. For this purpose, the starting materials are generally processed to give a kneadable slurry, which is extruded to give a honeycomb permeated by channels. Subsequently, the extruded honeycomb is calcined with solidification by a thermal treatment to give the finished unsupported catalyst.
Further known constituents of SCR-active catalysts are also zeolites. Zeolites, i.e. framework aluminosilicates, in some cases form a structure permeated by channels of a diameter in the order of magnitude of gas molecules and, due to their high specific surface area, are especially suitable for a selective catalytic reduction.
For instance, DE 198 54 5502 A1 discloses an SCR-active catalyst for degradation of nitrogen oxides in the presence of a reducing agent having an active composition comprising titanium dioxide and a zeolite, said zeolite being a hydrogen ion-exchanged, acidic zeolite.
GB 2 193 655 A also discloses a catalyst for degradation of nitrogen oxides by the SCR process. The catalyst composition of the catalyst specified therein comprises a titanium dioxide with low specific surface area and a copper-containing zeolite obtained by ion exchange. Preferred zeolites specified are mordenite, ZSM-5 and ferrierite.
Also known from EP 0 393 917 A2 is a catalyst for degradation of nitrogen oxides, the catalyst composition of which comprises a zeolite which, after ion exchange, contains copper and/or iron. Preferred zeolites specified are USY (Ultra Stabilized Y), beta and ZSM-20.
In addition, EP 0 219 854 A2 discloses a catalyst which comprises titanium dioxide in the anatase polymorph and an acid-stabilized zeolite in the hydrogen form or in the ammonium form.
Finally, U.S. Pat. No. 5,271,913 A discloses a catalyst for degradation of nitrogen oxides by the SCR process, the catalyst composition of which comprises a zeolite. The zeolite here is impregnated with cerium oxide or iron oxide. The catalyst specified is said to have a high stability with respect to sulfur-containing components. A preferred zeolite specified is a zeolite of the ZSM-5 type.
Zeolite catalysts can be produced either in the form of coated catalysts or in the form of unsupported catalysts. The configuration of a zeolite catalyst as a bulk material catalyst, particularly in the form of pellets, is also known per se.
The literature mentions an Fe ion-exchanged zeolite in particular with regard to its good SCR activity. For example, the publication “Ultra-Active Fe/ZSM-5 Catalyst For Selective Catalytic Reduction Of Nitric Oxide With Ammonia”, Gongshin Qi, Ralph T. Yang, Apple. Cat. B: Environmental 60 (2005) 13-22 studies the SCR activity of an Fe ion-exchanged ZSM-5 zeolite for NO, and conversion rates of NO close to 90% are achieved at temperatures above 350° C. in the presence of ammonia. The ion exchange of the zeolite of the ZSM-5 type studied is undertaken by impregnation by means of FeCl3. The calcination of the catalyst composition takes place under oxidizing atmosphere in air. By means of X-ray diffraction and electron spin resonance measurements, the oxidation state of the iron ion incorporated into the zeolite structure is determined to be +2 and/or +3. It is suspected that iron with the +2 oxidation state in particular is responsible for a high catalytic activity. During the SCR reaction studied, iron of the +2 oxidation state is gradually oxidized.
In the prior publication “Structure/Reactivity Correlation In Fe/ZSM5 For DENOx Applications. In-situ XAFS Characterization And Catalysis”, A. A. Battiston, J. H. Bitter, D. C. Koningsberger, Elsevier, an Fe ion-exchanged zeolite of the ZSM-5 type is also studied with regard to its SCR activity. In this case, particularly the coordination site of the iron ion incorporated into the zeolite structure is analyzed by means of X-ray spectroscopy methods. The ZSM-5 zeolite studied is ion-exchanged by means of FeCl3 sublimation. The SCR activity of the calcined catalyst composition is analyzed using butane and isobutane as reducing agents. The catalyst composition is studied by X-ray spectroscopy in each case after treatment with oxygen, carbon monoxide and isobutane. During the SCR reaction, the oxidation state of the iron ion incorporated is said to be reduced.